Magnet controller

ABSTRACT

A control system for a lifting magnet measures the lift current of the magnet and causes the drop-out current to reach a predetermined percentage of the lift current at interruption so that clean drops are provided, without adjustment, regardless of the condition of the circuit elements, the temperature of the lifting magnet, or the size of the magnet.

United States Patent 1 111 3,723,825 De Viney [4 1 Mar. 27, 1973 [54]MAGNET CONTROLLER 3,368,119 2/1968 Littwin ..317 157.5

75 l t T E.D V ,S H", l 1 men or i g 6 "my even x S Primary Examiner.l.D. Miller Assistant Examiner-Harry E. Moose, Jr. [73] Assignee: Square DCompany, Park Ridge, Ill. Atm e H rld J. Rathbun er a],

2 d: 1 1 72 l 2] 9 57 ABSTRACT 21 A l. N 219,020 1 pp 0 A control systemfor a lifting magnet measures the lift current of the magnet and causesthe drop-out current [52] U.S. Cl 317/123, 317/157.5 to reach apredetermined percentage of the lift ur- [51] Int. Cl ..H0lf 13/00 rentat interruption so that clean drops are provided, Field of Search DIG.without adjustment, regardless of the condition of the 295 circuitelements, the temperature of the lifting magnet, or the size of themagnet. [56] References Cited 12 Claims, 2 Drawing Figures UNITED STATESPATENTS 3,579,053 /1971 Littwin ..3l7/l57.5 X

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32 37 39 I58 50 5M 54 467 a5 9/ I061, /.97

. 94 /0/ /04 S 6/ 64 76 I24 5 7 40 WW /7 62 \96 m2 3/ 67 550/ l [22 [25/2a 552 m5 1 36 f30 l MAGNET CONTROLLER This invention relates to animproved system for, and a method of, controlling inductive devices, andmore particularly to an improved control system and method forautomatically effecting total demagnetization of inductive devices suchas lifting magnets in-a short time.

It is common practice to magnetize a lifting magnet by connecting it toa source of constant unidirectional voltage and to demagnetize themagnet by reversing the connections to the source through a fixed valueof resistance. The reverse current is interrupted automatically when themagnet flux is approximately zero.

To provide for a clean drop of a load of scrap iron or similar materialupon demagnetization of the magnet without leaving stickers anddribblers, it is necessary that the reverse current, upon itsinterruption, be of a value which does not leave any residual magnetismin the magnet. This magnitude of reverse current is dependent uponseveral variables. Among these are the size of the magnet, thetemperature of the magnet, variations in line voltage, and the conditionof circuit components such as contact surface condition.

Prior art devices utilize various means to fix the value of reversecurrent at which the drop cycle is terminated. Some use timing circuitsand start a fixed time period either from the beginning of the dropcycle or from the instant of current reversal and terminate the dropcycle at the end of the fixed time period. Other devices measure thevoltage across, or the current through, the magnet during the drop cycleto determine when the proper level of reverse current has been reached.Each of these devices terminates the drop cycle after the reversecurrent has reached a preselected value independent of variations incircuit conditions.

However, when the temperature of the magnet or other circuit conditionchanges, the magnetization curve for the magnet is altered; for example,a value of reverse current which will fully demagnetize a cold magnetwill leave residual magnetism in a hot magnet. For this reason, priorart circuits have included adjustments to compensate, in a hunt-and-peckmanner, for such variations. Unfortunately, since circuit conditions areconstantly changing, frequent adjustments are required and a significantamount of time is lost making these adjustments. Often, magnet operatorsprefer not to make the adjustment and either try to drop stickers anddribblers by shaking the magnet or just simply ignore them. Thisconsumes even more time and greatly decreases the efficiency of theoperation.

Attempts have been made to provide automatic compensation for individualfactors which change the required reverse demagnetization current. Oneof these comprises using a thermocouple embedded within the magnet sothat the magnet controller can provide automatic temperaturecompensation. However, no automatically adjusting system has been ableto compensate for the several varying conditions responsive for changesin magnetization.

l have discovered that the variations in magnetization of the magnet forwhich compensation is required are directly reflected in changes in thelift current through the magnet caused by these differing conditions andthat complete demagnetization may be effected, through the applicationof the correct magnitude of reverse current for each drop, by measuringthe magnitude of the lift current at the termination of the lift cycleand removing the reverse current when it reaches a predeterminedpercentage of the lift current at interruption of the lift circuit.

Accordingly, it is an object of this invention to provide an improvedmagnet controller and more particularly to provide a magnet controllerhaving means for measuring the lift current at the termination of thelift cycle and terminating the drop cycle when the reverse currentthrough the magnet reaches a predetermined percentage of the liftcurrent.

These and other objects and advantages of the present invention willbecome apparent from the following description in which reference ismade to the drawings wherein:

FIG. 1 is a schematic wiring diagram of a magnet control system inaccordance with this invention; and,

FIG. 2 is a schematic wiring diagram of the magnet control system ofFIG. 1 illustrating a preferred static discharge control unit therefor.

In general, the magnet demagnetizing method of this invention includesthe steps of measuring the lift current through the magnet at thetermination of the application of lift voltage, storing a first voltageproportional to lift current, applying reverse voltage to the magnet toproduce a reverse current in the magnet in a direction opposite that ofthe lift current, comparing a second voltage proportional to the reversecurrent with the first voltage, and disconnecting the reverse voltagewhen the second or reverse current proportional voltage bears apredetermined relationship to the first or lift current proportionalvoltage.

Referring now to FIG. 1, a magnet control circuit in accordance withthis invention is illustrated as connected to control the energizationof a magnet 11 from a direct current source.

A positive terminal 12P of the direct current source is connected by aconductor 14 through a normally open contact 15a of a lift contactor 15,a junction 16 and a normally open contact 17a of a drop contactor 17 toa junction 19, and by a conductor 20 through a resistor 21, a normallyopen drop contact 17b, a junction 22, a normally open lift contact 15band a junction 24 to a negative terminal 12N of the source. The mag net1 l is connected between the junctions l6 and 22 by conductors 25a and25b. Connected between the junctions 19 and 24 is a voltage dividercomprising seriesconnected resistors 26 and 27 having a junction 29therebetween.

A conductor 30 connects the positive terminal 12? of the direct currentsource through a junction 31, a junction 32 and anormally open auxiliarycontact of the drop contactor 17 to a junction 34.

A master switch 35 has a normally open contact 36 and a normally closedcontact 37. The master switch may be constructed to provide overlappingoperation for the contacts 36 and 37 when the master switch 35 is movedfrom its LIFT position to its DROP position so that the contact 37closes before the contact 36 opens during this operation. As can be seenby the schematic representation of the master switch 35, the contact 36is open in the DROP position and closed in theLlFT position of themaster switch 35 while the contact 37 is closed in the DROP position andopen in the LIFT position of the master switch.

Electrical connection of the master switch 35 is provided by a conductor39 connected from the junction 32 through the contact 37, a normallyopen auxiliary contact 15c of the lift contactor l and an operatingwinding 17w of the drop contactor 17 to the junction 29, and by aconductor 40 connected from the junction 31 through the master switchcontact 36 and an operating winding w of the lift contactor 15 to aconductor 41. A resistor 43 is connected in parallel with the winding15w, and the conductor 41 is connected between the junction 24 and astatic discharge control unit 42.

The static discharge control unit 42 is connected by a conductor 44through a resistor 45 to the junction 34, by a conductor 46 through aresistor 47 to the junction 34, by a conductor 49 to the junction 19, bya conductor 50 to the conductor 39 adjacent the positive side of thedrop contactor winding 17w and by a conductor 51 to the conductor 39adjacent the negative side of the drop contactor winding 17w.

Operation of the magnet control circuit of FIG. 1 will now be described.Although, for the purposes of this description, the overlapping masterswitch 35 is used, it should be understood that a push button, or othertype, magnet controller could be substituted because the requiredcontact overlap is provided by the connection of the resistor 43 acrossthe lift contactor winding 15w to delay opening of the lift contacts15a, b, 0 until after closure of the drop contacts 17a, b, c.

Initially, with the master switch 35 in its DROP position, the contact36 is open so that the lift contactor winding 15w is not energized andthe lift contacts 15a and 15b are open. Although master switch contact37 is closed, auxiliary lift contact 15c is open and the drop contactorwinding 17w is therefore not energized so that the drop contacts 17a and17b are open. Thus, initially, the magnet 11 is disconnected from thedirect current source. 7 7

When the master switch 35 is moved to its LIFT position, the masterswitch contact 36 closes and the master switch contact 37 opens. Theclosing of the contact 36 completes a circuit from the positive terminal121 through the conductor 30, the junction 31, the contact 36, theconductor 40, the lift contactor winding 15w and the conductor 41 to thenegative terminal 12N. This circuit energizes the lift contactor windingwhich causes closure of the contacts 15a, 15b and 15p. The closing ofcontacts 15a and 15b connects the magnet 11 directly to the directcurrent source so that current flows through the magnet 11 in thedirection of an arrow 52 to magnetize the magnet 11 in a well knownmanner. The closing of the contact 150 does not affect LIFT operationsince the master switch contact 37, serially connected with the contact15c, is open.

Demagnetization of the magnet 11 is accomplished by supplying a reversecurrent of suitable value to the magnet. When the master switch 35 ismoved to the DROP position, the master switch contact 37 closes beforethe contact 36 opens. The closing of the contact 37 energizes thewinding 17w through a circuit from the positive voltage terminal 12?through the conductor 30, the junction 32, the contact 37, the conductor39, the still closed lift contact 15c, the winding 17w and the resistor27 tothe negative voltage terminal 12N. The drop contactor winding 17wcauses closure of contacts 17a, 17b and 17c. Closing of the contacts 17aand 17b places the resistor 21 and the serial combination of resistors26 and 27 each in parallel with the magnet 11. Closure of contact 170provides a power supply input for the static discharge control unit 42from the junction 32 through the resistor 45 and conductor 44 andcompletes a holding circuit for the drop contactor winding 17w throughthe resistor 47, conductor 46, static discharge control unit 42 and theconductor 50 as will hereinafter be explained. I

When the master switch contact 36 opens, the lift contactor winding 15wis de-energized and contacts 15a, 15b and 15c open. Now voltage isapplied to the magnet 11 through a circuit from the positive voltageterminal 121 through the resistor 21, the conductor 20, the contact 17b,the conductor 25b, the magnet 11, the conductor 25a, the contact 17a,the resistor 26, and the resistor 27 to the negative voltage terminal12N. Opening of the contact leaves the winding 17w energized solelythrough the static discharge control unit 42 so that termination ofreverse current through the magnet l 1 can be controlled thereby.

Although the polarity of the voltage applied to the magnet 11 isreversed by the opening of the contacts 15a and 15b, the currentthroughthe magnet 11 after the opening of the contacts 15a and 15b isgenerally equal to, and in the same direction as, the lift current justbefore the drop contacts 17a and 17b closed becauseof the highlyinductive nature of the magnet 11. Accordingly, there is a voltage dropacross the resistor 27, with the junction 24 positive with respect tothe junction 29, the voltage drop having a magnitude directlyproportional to the lift current at the instant the contacts 15a and 15bopen. This voltage is transmitted by the conductor 41 and conductors 39and 51 to the static discharge control unit 42 which includes means forstoring this voltage in a manner to be 1 described.

As application of reverse voltage continues, the current through themagnet 11 diminishes and eventually begins to flow in a directionopposite that indicated by the arrow 52. Thereupon, a voltage is placedacross the resistor 26 with the junction 19 positive with respect to thejunction 29. This voltage is of a magnitude proportional to that of thereverse current through the magnet and is transmitted through theconductor 49 and through the serially connected conductor 39 and 51 tothe static discharge control unit 42. The voltage across the resistor 26is compared by the static discharge control unit 42 with the previouslystored voltage from across the resistor 27. When the relative magnitudesof these voltages indicate that the reverse current has reached apredetermined percentage of the terminal 1 value of lift current, thestatic discharge control unit 42 de-energizes the drop contactor winding17 w. This may be accomplished by completing a circuit by-passing thewinding 17w through the resistor 47, the conductor 46 and the conductor51.

De-energization of the winding 17w opens the contacts 17a, 17b and 17c.Opening of the contact 17c removes power from the static dischargecontrol unit 42. Opening of the contacts 17a and 17b disconnects themagnet 11 from the direct current source so that the magnet controlcircuit is turned off and the magnet 1 1 is completely de-energized.

As can be seen from the previous discussion, the static dischargecontrol unit 42 may be any circuit which will store a voltageproportional to the final value of lift current, compare this voltage toa second voltage proportional to reverse current, and de-energize thedrop contactor winding when the reverse current reaches a predeterminedpercentage of the final value of lift current. A preferred embodiment ofsuch a static discharge control unit 42 is illustrated in FIG. 2 and isshown connected as part of the magnet control system of FIG. 1. Althoughthe'representations of circuit components have been rearranged toclarify description, the magnet control system as illustrated in FIG. 2is electrically the same as the representation of FIG. 1. Accordingly,the numbers used in FIG. 1 have been retained in the description of FIG.2. The conductor 41 from the junction 24 is connected to a terminal 41T.The conductor 44 is connected to a terminal 44T. Conductors 46 and 50are respectively connected to terminals 46T and 50T which have a diode54 connected therebetween. Terminating the conductor 49 from thejunction 19 is a terminal 49T while the conductor 51, also connected tothe conductor 39 leading from the junction 29, is connected to aterminal 51T which is connected to serve as the system common or ground.The terminal SIT is also connected to a common conductor 55. v

Connected between the terminal 44T and the conductor 55 are a diode 56and a Zener diode S7 serially connected by a conductor 59. A capacitor60 is connected in parallel with'the Zener diode 57. The diode 56, Zenerdiode 57 and capacitor 60 combine to function as a power supply for thestatic discharge control unit 42.

A diode 61 is connected between the terminal 50T and a branch 55a of thecommon conductor 55 while a conductor 62 connects a thyristor 64 betweenthe terminal 46T and the common conductor branch 55a. The gate of thethyristor 64 is connected through a resistor 65 to a junction 66 whichis in turn connected by a resistor 67 to the common conductor branch55a.

The conductor 59, between the diode 56 and the Zener diode 57, isconnected by a conductor 69 through a resistor 70 and a unijunctiontransistor 71 to the junction 66. A conductor 72 connects the conductor59 to the common conductor branch 55a through a of an NPN transistor 86.The junction 84 is connected to the base of the transistor 80.Conductors 87 and 89 connect the conductor 59 to the collectors of NPNtransistors 90 and 91, respectively. The emitter of the transistor 91 isconnected to the base of the transistor 90 and the emitter of thetransistor 90 is connected by a conductor 92 through a junction 94 and adiode 95 to the emitter ofthe transistor 86. Connected between thejunction 94 and the common conductor 55 is a resistor 96.

The base of the transistor 91 is connected by a conductor 97 through adiode-99 and a diode 100 to the common conductor 55. A capacitor 101 isconnected between the conductor 97, adjacent the transistor 91, and thecommon conductor 55 by a conductor 102. The conductor 97 is alsoconnected, at a point between the diodes 99 and 100, by a conductor 104through a resistor 105 to the common conductor 55 and by a conductor 106through a resitor 107 to the terminal 41T.

Serially connected between the terminals 49T and 51T by a conductor 109are a diode 110, a resistor 111, a junction 112 and a resistor 114. Aconductor 115 connects the junction 112 through a threshold means suchas a silicon unilateral type switch (SUS) 116 to the junction 66. Acapacitor 117 is connected between the conductor 115 and the commonconductor 55.

The terminal 49T is connected by a conductor 119 through a resistor 120,a junction 121, and a resistor 122 to ajunction 124. The junction 124 isconnected to the base of the transistor 86 and is connected through aresistor 125 and, by a parallel path, through a diode 126 to the commonconductor 55.

Joining the junction 121 to the common conductor 55 is a conductor 127which serially connects a diode 129, ajunction 130, a resistor 131,ajunction 132 and a resistor 134. A thyristor 135 is connected betweenthe junction and the common conductor 55 by a conductor 136 while aconductor 137 connects the gate of the thyristor to the junction 132through an SUS 139.

Operation of the magnet control circuit utilizing the static dischargecontrol unit circuit illustrated in FIG. 2 will now be described. Whenthe master switch is moved to its LIFT position, the magnet is energizedthrough a circuit from the positive voltage-terminal 12P through theconductor 14, the contact 15a, the conductor 25a, the magnet 11, theconductor 25b, the conductor 20 and the contact 15b to the negativevoltage terminal 12N. Voltage is not applied across either the resistor26 or the resistor 27 and both the master switch contact 37 and thecontact are open so that no input voltage is applied to the staticdischarge control unit 42.

When the master switch is moved to the DROP position, energizing thewinding 17w of the drop contactor while thewin ding 15w of the liftcontactor remains energized, the drop contacts 17a and 17b close placingthe serial combination of resistors 26 and 27 and the resistor 21 inparallel with the magnet 11. This places a voltage drop across theserial combination of resistors 26 and 27 equal to the voltage of thedirect current source. If the ohmic values of resistors 26 and 27 areequal, this places a voltage drop across each of the resistors 26 and 27which is one-half of the line voltage with the junction 19 positive withrespect to the junction 29 and the junction 24 negative with respect tothe junction 29. It should be kept in mind at this time that thejunction 29 is connected through conductors 39 and 51 to the commonconductor 55 of the static discharge control unit 42 at the terminal 51Tso that the voltage drop across the resistors 26 and 27 are reflected bythe voltage at the junctions 19 and 24, respectively. The junctions l9and 24 are connected through the conductors 49 and 41, respectively, tothe corresponding input terminals of the static discharge control unit42 so that these voltages will be applied thereto.

Energization of the winding 17w causes closure of the contact 17c which,through the resistor 45 and the conductor 44, energizes the power supplycircuit comprising the terminal44T, the diode 56, the conductor 59 andthe parallel-connected combination of the capacitor 60 and the Zenerdiode 57. Closure of the contact 17c also completes a holding circuitfor the winding 17w through the resistor 47, the conductor 46, theterminal 46T, the diode 54 and the terminal SOT to the conductor 50 andwinding 17w.

The negative voltage at the junction 24 is applied through the conductor41 to the terminal 411 of the static discharge control unit 42 andapplied through the resistor 107 to the conductor 97. This voltage isthen applied to the common conductor 55 through the diode 100 and isblocked from the capacitor 101 and the base of the transistor 91 by thediode 99. Thus, the voltage drop across the resistor 27 has no effectduring this portion of circuit operation. The voltage across theresistor 26 is applied through the conductor 49 to the terminal 49T. Thevoltage at the terminal 49T is applied across the series combination ofthe diode 110, resistor 111 and resistor 114. The resistor 111 and 114form a voltage divider which places a predetermined voltage at thejunction 112 when the voltage drop across the resistor 26 is aboutone-half of line voltage. The capacitor 117 provides a time delay beforethe voltage at the junction 112 causes the SUS 116 to break over intoconduction. However, the period of overlap of the master switch contacts36 and 37', during which this magnitude of voltage is applied to theterminal 49T, is shorter than the time delay provided by the capacitor117. Therefore, this portion of the circuit does not affect normal dropoperation.

The voltage at the terminal 49T is also applied, through the resistor120,-to the junction 121. This voltage is placed, through the diode 129,across the voltage divider comprising the resistors 131 and 134. Duringthis period of lift and drop contact overlap, the voltage sistors 122and 125 so that the voltage at the junction 124, and accordingly at thebase of the transistor 86, is too small to cause the transistor 86 toconduct. Thus, operation of the magnet control circuit of FIG. 1 isgenerally unaffected by the static discharge control unit -42 during theperiod of overlap of the master switch contacts 36 and 37.

After the contact overlap period, the contact 36 opens de-energizing thewinding 15w. The resistor 43, which is connected across the winding 15w,further delays opening of the lift contactor to ensure the closing ofthe drop contacts 17a and 17b before the lift contacts 15a and 15b openso that arcing of the lift contacts may be prevented, as is well knownto those skilled in the art.

As has been previously indicated, after the opening of the lift contacts15a and 15b, the magnet 11 is energized by a circuit from the positivevoltage terminal 12? through the resistor 21, contact 17b, conductor25b, magnet 11, conductor 25a, contact 17a, resistor 26 and resistor 27to the negative voltage terminal 12N.

dicated by the arrow 52 and is approximately equal to the magnitude ofthe current through the magnet 11 just before the closing of the DROPcontacts. Because of this, there is a voltage drop across the .resistor26 with the junction 19 negative with respect to the ju'nction 29 and avoltage drop across the resistor 27 with junction 24 positive withrespective to the junction 29.

The negative voltage at the junction 19 is applied through the conductor49 to the terminal 49T of the static discharge control unit 42 and thenapplied through the conductor 119 and the resistors 120 and 122 to thediode 126 which is poled to transmit this voltage and prevent thebiasing into conduction of the transistor 86 during this portion of thedrop cycle. The diodes and 129 are poled to block this voltage. Thus,the voltage across the resistor 26 does not affect this portion of thedrop cycle.

' The positivevoltage at the junction 24 is appliedthrough the magnet 11just before initiation of the drop cycle. The voltage stored by thecapacitor 101 biases the transistors 91 and 90, which are connected tooperate as a Darlington transistor emitter follower, into conduction toplace a voltage substantially equal to that stored by the capacitor 101across the resistor 96.

The high gain of the emitter follower circuit maintains the voltagelevel at the junction 94 without discharging the capacitor 101 duringthe time between the opening of the lift contacts 15a and 15b and thebuildup to the proper level of reverse current through the magnet l 1.

Continued application of voltage to the magnet 11 with the junction 22positive relative to the junction 16 decreases the current which isflowing through the magnet 11 in the direction of the arrow 52. This,however, has no effect on the voltage stored by the capacitor 101 in thestatic discharge control unit 42 due to the blocking action of the diode99. The current through the magnet 11 decreases to zero and thenreverses and starts to build up in a directionopposite to that of thearrow 52. This causes a voltage drop across the resistors 26 and 27 withthe junction 19 positive with respect to the junction 29 and thejunction 24 negative with respect to the junction 29.

The negative voltage at the junction 24 is applied through the conductor41, the terminal 41T, the resistor 107 and the diode 100 to the commonconductor 55. The voltage is blocked by the diode 99 so that it does notaffect the voltage stored by the capacitor 101. The positive voltage atthe junction 19 is applied through the conductor 49, the terminal 49T,the through the resistors and 122 to the junction 124 at the base of thetransistor 86. This voltage is blocked by the diode 126 so that thejunction 124 is at a voltage which is proportional to the magnitude ofreverse current through the magnet 11. Although the positive voltage atthe input terminal 49T is not blocked by the diode 110 or the diode 129,the voltage levels established at the junctions 112 and 132 are toosmall to trigger either the SUS 116 or the SUS 139, respectively.

When the voltage at the junction 124, which is proportional to thereverse current through the magnet 11, bears a predetermined ratio tothe voltage at the junction 94, which is proportional to the liftcurrent, the base-emitter voltage of the transistor 86 reaches amagnitude sufficient to bias the transistor 86 into conduction.Thereupon, current flows from the conductor 59, in the power supplycircuit, through the resistor 82, the resistor 85, the transistor 86,the diode 95 and the resistor 96 to the common conductor 55 andestablishes a voltage at the junction 84 which causes the transistor 80to conduct. This completes a charging circuit for the capacitor 76through the transistor 80 and the resistor 79 and rapidly charges thecapacitor 76 to the trigger voltage of the unijunction transistor 71.The voltage thereby produced at the junction 66 upon conduction of thetransistor 71 generates a pulse that is fed through the resistor 65 tothe gate of the thyristor 64 causing it to conduct.

The conduction of the thyristor 64 completes a circuit from the inputterminal 46T through the thyristor 64, conductor 62 and the commonconductors 55a and 55 to the input terminal SlT so that the holdingcircuit for the drop contactor winding 17w is by-passed and currentflows from the conductor 46 through the static discharge control unit 42to the conductor 51 instead of the conductor 50. The winding 17w isde-energized and the contacts 17a, 17b and 17c consequently open toterminate the drop cycle. The free wheeling diode 61 provides a currentpath for the induced voltage in the winding 17w when the thyristor 64 isconducting.

Because the reverse current through the magnet 11 at the end of the dropcycle always reaches the proper magnitude determined by the lift currentto completely demagnetize the magnet, a clean drop is always providedfor the magnet without any adjustment being required.

It is desirable to provide circuitry which terminates the drop cycleafter a fixed period of time so that the drop cycle will be terminatedeven if the time required for the reverse current buildup in the magnetis greater, due to circuit malfunction, than the fixed time period or ifthe controller is cycled without a magnet. For this purpose, theresistor 74 is connected between the power supply conductor 59 and thejunction 75 to provide an independent charging circuit for the capacitor76. With the proper selection of capacitors 76 and resistor 74, the timerequired to charge the capacitor 76 to the trigger voltage of theunijunction transistor 71 can be preset to be just greater than thelongest time constant of any magnet which will be operated by thecontrol system. When the drop contact 17c closes at the beginning of thedrop cycle, thereby energizing the power supply portion of the staticdischarge control unit 42, the capacitor 76 is charged slowly throughthe resistor 74. If the buildup of reverse current is not rapid enoughto turn on the transistor 86 during the charging period of the capacitor76, the capacitor 76 reaches the triggering voltage of the unijunctiontransistor 71 to gate the thyristor 64 on and terminate the drop cycleas has been previously described.

If, for any reason, the master switch is moved to the LIFT positionbefore the completion of the drop cycle, the drop cycle must beterminated to prevent damage to circuit components. Moving the masterswitch to the LIFT position closes the master switch contact 36 andenergizes the lift contactor winding 15w to close the contacts 15:: and15b. If the drop cycle has not been completed, the drop contactorwinding 17w is still energized and the contacts 17a and 17b are closedso that the serially connected resistors 26 and 27, as well as theresistor 21, are connected in parallel with the magnet 11 across thesource. Thus, the voltage drop across the resistor 26 is about one-halfof the source voltage with the junction 19 positive with respect to thejunction 29. This voltage is applied through the conductor 49 to theterminal 49T and from there through the resistor to the junction 121.The voltage is not blocked by the diode 129 and causes the thyristor toconduct and prevent this high voltage from being applied to thetransistor 86. Because the voltage at the terminal 49T is not blocked bythe diode 110, it produces a voltage at the junction 112 sufficient tobreak over the SUS 116 into conduction. After the time delay provided bythe capacitor 117, the SUS 116 conducts producing a voltage at thejunction 66 which gates on the thyristor 64 to terminate the drop cyclein the manner previously described. This leaves the magnet connected tothe positive voltage terminal 12? and negative voltage terminal 12N ofthe direct current source through the lift contactors 15a and 15b,respectively.

A magnet control system is thus provided which measures the lift currentat the termination of the lift cycle and terminates the drop cycle whenthe reverse current through the magnet has reached a predeterminedpercentage of the lift current. The drop cycle also terminates withinfixed time period after initiation of the drop cycle or when a liftcycle is started before the completion of the drop cycle. It should beclear that, although the magnet controller of this invention has beendescribed in connection with a lifting magnet, it may be utilized forcontrolling any electromagnet or for de-energizing any inductive devicewhich would otherwise leave an undesirable residual magnetic field.

Although, in the preferred embodiment, the magnitudes of the liftcurrent and reverse current are determined by measuring the voltage dropacross resistors 26 and 27, it should be understood that other means forproviding voltages proportional to these currents may be used withoutdeparting from the spirit and scope of this invention. It has been foundin practice that, for a lifting magnet, the reverse current shouldpreferably be interrupted when it reaches a magnitude of about 15percent of the lift current because, although magnet flux variessomewhat with the type of load, such a value provides a clean drop forall types of load.

Iclaim:

1. In a control system for selectively connecting and disconnecting anelectromagnet to and from a source of power, said control systemcomprising switching means for selectively connecting and disconnectingthe electromagnet directly to and from the source of power to controlthe flow of a lift current through the electromagnet, resistance means,and reverse switching means for connecting the electromagnet to thesource through the resistance means to cause a reverse current to flowthrough the electromagnet upon cessation of the lift current, theimprovement comprising control means for measuring the lift current,measuring the reverse current, and causing cessation of the reversecurrent through the electromagnet when the magnitude of the reversecurrent bears a predetermined relationship to the magnitude of the liftcurrent.

2. A control system as in claim 1 wherein said reverse switching meansincludes a drop contact which closes upon operation of said reverseswitching means and said switching means includes a lift contact whichopens after the drop contact closes, and said control means includesmeans'for measuring the lift current after the lift contact opens.

3. A control system as in claim 1 wherein said control means includesmeans for causing cessation of the reverse current through theelectromagnet when the magnitude of the reverse current reaches apredetermined percentage of the magnitude of the lift current.

4. A control system as in claim 1 wherein said control means includesmeans responsive to one voltage across the resistance means formeasuring said lift current and 'means responsive to an other voltageacross the resistance means for measuring said reverse current.

5. A control system as in claim 4 wherein said re sistance meansincludes a first portion and a second portion, said one voltageresponsive means is connected across said first portion and said othervoltage responsive means is connected across said second portion.

6. A control system as in claim 4 wherein said means responsive to saidone voltage includes storage means for storing a first voltageproportional .to the magnitude of the lift current and said controlmeans includes comparison means for comparing a second voltageproportional to the magnitude of the reverse current with said first,voltage, means responsive to the comparison means for producing anoperating signal when said second voltage bears a predeterminedrelationship to said first voltage, and terminating means responsive tothe operating signal for interrupting the reverse current through theelectromagnet.

7. A control system as in claim 6 wherein said storage means is acapacitor.

8. A control system as in claim 4 wherein said means responsive to saidone voltage includes storage means for storing a first voltageproportional to the magnitude of the lift current and said control meansincludes comparison means for comparing a second voltage proportional tothe magnitude of the reverse current with said first voltage and meansresponsive to the comparison means for producing an operating signaldependent upon the relationship said second voltage bears to said firstvoltage, and terminating means responsive to the operating signal forinterrupting the reverse current through the electromagnet when apredetermined relationship between the first and second voltages isreached.

9. A control system as in claim 8 wherein said storage means is acapacitor.

10. A control system as in claim 8 wherein said controlmeans includestiming means activated to initiate a timing cycle upon connection of theelectromagnet to the source through said resistance means by saidreverse switching means and operative to produce an operating signalafter a time delay, and overlap means activated to initiate a timingcycle when the electromagnet is connected directly to the source ofpower by said switching means while connected to the source through saidresistance means by said reverse switching means and operative toproduce an operating signal after a time delay, and wherein saidterminating means is responsive to each of the operating signal of thetiming means and the operating signal of the overlap means for stoppingthe flow of reverse current through the electromagnet.

11. The method of demagnetizing an electromagnet connected to a sourceof power to apply a lift voltage causing a lift current to flow throughthe electromagnet, said method comprising the steps of terminatingapplication of the lift voltage, applying reverse voltage to the magnetto produce a reverse current in the electromagnet in a directionopposite that of the lift current, and disconnecting the reverse voltagewhen the reverse current bears a predetermined relationship to the liftcurrent.

12. A method of demagnetizing an electromagnet connected to a source ofpower to apply a lift voltage causing a lift current to flow through theelectromagnet, said method comprising the steps of terminatingapplication of the lift voltage, storing a first voltage proportional tolift current, applying reverse voltage to the electromagnet to produce areverse current in the magnet in a direction opposite that of the liftcurrent, providing a second voltage proportional to the reverse current,comparing said first and second voltages, and disconnecting the reversevoltage when the firstvoltage bears a predetermined relationship to thesecond voltage.

1. In a control system for selectively connecting and disconnecting anelectromagnet to and from a source of power, said control systemcomprising switching means for selectively connecting and disconnectingthe electromagnet directly to and from the source of power to controlthe flow of a lift current through the electromagnet, resistance means,and reverse switching means for connecting the electromagnet to thesource through the resistance means to cause a reverse current to flowthrough the electromagnet upon cessation of the lift current, theimprovement comprising control means for measuring the lift current,measuring the reverse current, and causing cessation of the reversecurrent through the electromagnet when the magnitude of the reversecurrent bears a predetermined relationship to the magnitude of the liftcurrent.
 2. A control system as in claim 1 wherein said reverseswitching means includes a drop contact which closes upon operation ofsaid Reverse switching means and said switching means includes a liftcontact which opens after the drop contact closes, and said controlmeans includes means for measuring the lift current after the liftcontact opens.
 3. A control system as in claim 1 wherein said controlmeans includes means for causing cessation of the reverse currentthrough the electromagnet when the magnitude of the reverse currentreaches a predetermined percentage of the magnitude of the lift current.4. A control system as in claim 1 wherein said control means includesmeans responsive to one voltage across the resistance means formeasuring said lift current and means responsive to an other voltageacross the resistance means for measuring said reverse current.
 5. Acontrol system as in claim 4 wherein said resistance means includes afirst portion and a second portion, said one voltage responsive means isconnected across said first portion and said other voltage responsivemeans is connected across said second portion.
 6. A control system as inclaim 4 wherein said means responsive to said one voltage includesstorage means for storing a first voltage proportional to the magnitudeof the lift current and said control means includes comparison means forcomparing a second voltage proportional to the magnitude of the reversecurrent with said first voltage, means responsive to the comparisonmeans for producing an operating signal when said second voltage bears apredetermined relationship to said first voltage, and terminating meansresponsive to the operating signal for interrupting the reverse currentthrough the electromagnet.
 7. A control system as in claim 6 whereinsaid storage means is a capacitor.
 8. A control system as in claim 4wherein said means responsive to said one voltage includes storage meansfor storing a first voltage proportional to the magnitude of the liftcurrent and said control means includes comparison means for comparing asecond voltage proportional to the magnitude of the reverse current withsaid first voltage and means responsive to the comparison means forproducing an operating signal dependent upon the relationship saidsecond voltage bears to said first voltage, and terminating meansresponsive to the operating signal for interrupting the reverse currentthrough the electromagnet when a predetermined relationship between thefirst and second voltages is reached.
 9. A control system as in claim 8wherein said storage means is a capacitor.
 10. A control system as inclaim 8 wherein said control means includes timing means activated toinitiate a timing cycle upon connection of the electromagnet to thesource through said resistance means by said reverse switching means andoperative to produce an operating signal after a time delay, and overlapmeans activated to initiate a timing cycle when the electromagnet isconnected directly to the source of power by said switching means whileconnected to the source through said resistance means by said reverseswitching means and operative to produce an operating signal after atime delay, and wherein said terminating means is responsive to each ofthe operating signal of the timing means and the operating signal of theoverlap means for stopping the flow of reverse current through theelectromagnet.
 11. The method of demagnetizing an electromagnetconnected to a source of power to apply a lift voltage causing a liftcurrent to flow through the electromagnet, said method comprising thesteps of terminating application of the lift voltage, applying reversevoltage to the magnet to produce a reverse current in the electromagnetin a direction opposite that of the lift current, and disconnecting thereverse voltage when the reverse current bears a predeterminedrelationship to the lift current.
 12. A method of demagnetizing anelectromagnet connected to a source of power to apply a lift voltagecausing a lift current to flow through the electromagnet, said methodcomprising the steps of terminating applicatIon of the lift voltage,storing a first voltage proportional to lift current, applying reversevoltage to the electromagnet to produce a reverse current in the magnetin a direction opposite that of the lift current, providing a secondvoltage proportional to the reverse current, comparing said first andsecond voltages, and disconnecting the reverse voltage when the firstvoltage bears a predetermined relationship to the second voltage.